Tonight, the supreme court of the united states says it is time to decide one of the most highly charged issues in this country. Gay marriage. They have agreed to rule on the question, do same sex couples have a right to get married under the u.S. Constitution? It will be a history-making decision, and surprisingly, it's an 83-year-old woman at the center of this case. And abc's expert on the supreme court, terry moran, is here right now. Terry? Reporter: Well, diane, constitut constitutionally, it doesn't get any bigger than this. The country's charged so much on this issue. The real question is, how much? And, like so many landmark cases, as you point out, this one begins with the story of one american, this case is going to go down in history as edie windsor versus the united states. Edie windsor -- 83 years old -- is a widow now. But she had 42 happy years with the love of her life -- thea spyer. She was beautiful. She was smart as hell, she was wonderful altogether. Reporter: Edie and thea were married -- but when thea died in 2009, edie got socked by the irs with $363,000 in estate taxes -- which no widow in a straight marriage would have to pay. Today, the supreme court decided to hear edie's case challenging the law she says discriminates against couples like her and thea. The defense of marriage act, which defines marriage under federal law as the union of one man and one woman. The justices will also decide whether states have the power to ban gay marriage. And 30 states have laws that do just that. It's been a breathtaking year of charge on the issue. President obama, in a switch from 2008, announced his support for gay marriage in a may interview with abc's robin roberts. For me personally, it is important for me to go ahead and affirm, I think same sex couples should be able to get married. Reporter: On election day last month, voters approved gay marriage at the ballot box for the first time, after dozens of defeats, when maine, maryland and washington state legalized it. But now, it all comes down to the justices of the supreme court, and they are generally conservative on social issues, though they're closely divided. For opponents of gay marriage, the very fabric of our society is at stake. The fundamental reason why marriage is treated as a public institution rather than a purely private relationship is because it serves the intere of society and serves the interest of children. Reporter: Edie windsor thinks it's simper than that. If you could talk to the supreme court as they consider the case, what would you tell them? It's a marriage that anyone would want, gay or straight. We had a wonderful life together. And terry, right here again. So, when will the court rule on this and do you have any sense at all which may they may go? Reporter: Two cases here, edie's and one out of colorado. They will hear decisiabout it in march. If I were a betting man, I would say the court is likely to strike down that federal law, which sets the federal standard of marriage, but I don't think they're going to change the laws of all the country. So, it will go back to the states. We'll have a patchwork. But that's just an informed guess. Okay. Thank you for guessing for us. And, again, item us the date when they'll decide, roughly? Reporter: At the end of june. June. Okay, thank you terry moran.

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